Shakespeare's diction in Sonnet CXXX mocks the language of the Petrarchan sonnet that employs elegant comparisons. In contrast, with this sonnet the poet describes what his love is not, thus providing a parody of the sonnet model.
The speaker of Sonnet CXXX describes his love in the most unflattering language: Her eyes are not stunning, her lips are not red, and her breasts are not white, but are, instead, dun-colored. The speaker further flaunts the conventions of the sonnet sequence with such lines as these:
If hairs be wires, black wires grow on her head. . .
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
Rather than writing about his love as an ideal as in the traditional sonnet, Shakespeare uses diction that indicates his use of parody. This diction also serves another purpose as it lends verity to his ideas. For, when he declares his love for his "mistress," his words ring much truer than if he were to use the flowery language of a traditional Petrarchan sonnet:
And yet, by heaven, I think my love as rare
As any she belied with false compare.
Indeed, the use of unflattering diction indicates the reality of the speaker's love when he declares the rarity and genuineness of his feelings for his "mistress," who lacks many of the stellar qualities of the Petrarchan mistress.